Hi Rajan,
In case of Alternative Resources with Work Center classification, only by BAdI can the processing times of the alternative modes be modified (during PDS transfer).
If Alternative Sequences are created in the Routing, there is a BAdI solution in an SAP note that allows to transfer them to APO, and in this case the operation durations are translated to the alternative modes as maintained.
Also, given that PP heuristics (SAP_PP_002) are mostly used with Infinite Scheduling, overload in a given mode will not automatically cause the activity to shift to the other mode. This can be achieved later on with DS heuristics. One remark here is the fact that the system will not choose the mode with highest priority, but rather select the fastest mode (e.g. an activity takes 60s with Mode 1 Priority A and 45s with Mode 2 Priority B - if both resources are free, Mode 2 Priority B will be chosen).
Regards,
Tiago